1) Combine two tablespoons olive oil, three tablespoons balsamic vinegar, orange juice, two teaspoons minced garlic, chilli flakes, and 1/4 teaspoon each salt and pepper in a bowl and whisk.
2) Place steak and mushrooms in a glass baking dish or sealable plastic bag and pour marinade on top. Marinate in the refrigerator for at least one or up to four hours.
3) Combine remaining two tablespoons olive oil, three tablespoons vinegar, one teaspoon minced garlic, water, mustard, and remaining 1/4 teaspoon each salt and pepper. Whisk until incorporated.
4) Place French beans in a steamer basket over a few inches of boiling water and steam until tender-crisp, about 3 minutes. Allow to cool in the refrigerator, or to cool quickly, drain and plunge in a bowl of ice water for 2 minutes. Drain and reserve.
5) Spray a grill or grill pan with non-stick cooking spray and heat over medium-high heat. Remove steak and mushrooms from marinade, and discard marinade. Grill steak until medium-rare and mushrooms until cooked through but still juicy, about 4 to 5 minutes per side for both steak and mushrooms.
6) Remove to a cutting board and let rest about 10 minutes. Cut steak across the grain into 1/2-cm slices; cut mushrooms into 1/2-cm slices.
7) To assemble salad, toss steak slices, mushroom slices, spinach leaves, tomatoes, French beans and onions with dressing. Divide among four plates.